One process that is performed during the fabrication of a semiconductor device is to form a dielectric film, such as silicon oxide (SiO2) or silicon nitride (SiN2), on a semiconductor substrate. A thermal chemical vapor deposition (CVD) process is sometimes used to deposit such films. In this process, reactive gases are supplied to a substrate surface, and heat-induced chemical reactions take place to produce the desired film.
Various combinations of reactive gasses have been used to produce silicon oxide and silicon nitride dielectric films. These combinations generally include a silicon source and an oxidizing or nitridizing species. For example, dichlorosilane (SiH2Cl2), also referred to as DCS, has been used as a silicon source, and nitrous oxide (N2O) has been used as an oxidizing species. Ammonia (NH3) has been used as a nitrogen source.
In a multi-wafer CVD system, multiple semiconductor wafers are loaded into a reaction tube, the tube is heated, and the reactive gasses are introduced at an entry point at a one end of the tube. The gasses pass over the wafers and through the tube to an exit point. During the reaction, the DCS may be subjected to dissociation of hydrogen to produce dichlorosilane (SiCl2) and hydrogen (H2) as shown in the following equation (1).SiH2Cl2→SiCl2+H2  (1)At a high temperature, the chemical bond between the silicon atom and both the two chlorine atoms and the hydrogen atom are subjected to the elimination of hydrogen and chlorine to make the silicon atom into a new silicon atom terminated by the sole chlorine atom. Meanwhile, thermal decomposition of the N2O occurs all along the tube, and the thermal decomposition of the N2O is catalyzed by the chlorine by-product.
At the entry end of the reaction tube, the amount of chlorine gas available from the DCS decomposition is relatively small. Therefore, the decomposition rate of the N2O is simply characteristic of a thermally-driven decomposition. However, further along the tube, a higher concentration of chlorine exists as a by-product of the DCS decomposition. The higher abundance of chlorine increasingly interacts with and enhances the decomposition and reaction of the N2O, which increases the oxidation reaction rate. Accordingly, at the entry end of the tube, a more nitrogen rich environment exists, and toward the exit end of the tube, a more chlorine rich environment exists. This causes a non-stable reaction stoichiometry and rate from one end of the tube to the other. The result is that oxide films deposited near the entry end of the tube are thinner than oxide films deposited further down the tube.
What are needed are methods and apparatus for depositing dielectric layers in a more stable manner. Further needed are methods and apparatus for stabilizing the reaction stoichiometry and the reaction rate in various CVD systems, including CVD systems that utilize reaction tubes.